1. Field of the Invention
The present invention relates to polyurethane resins which are beneficial as material producing a polyurethane resin coat with, in particular, an excellent weather resistance.
2. Description of the Prior Art
Polyurethane resins are classified into two types by the kind of isocyanate compounds contained in them--the "yellowing" and "non-yellowing" types. Isocyanates which have hitherto been in use as ones giving polyurethane resins of the non-yellowing type include: such aliphatic isocyanates as hexamethylenediisocyanate, isophoronediisocyanate, 2,2,4-trimethylhexamethylenediisocyanate, dicyclohexylmethanediisocyanate, etc.; and xylilenediisocyanate, etc. Whilst non-yellowing type polyurethane resins obtained by causing these isocyanates to react with polyols have been employed as coating materials, there still remained many problems to be solved in using them for such purpose.
One of the problems is their toxicity. Since isocyanate compounds are substances which are chemically highly reactive, they are very dangerous when used by persons who are of an allergic constitution or have weak respiratory organs. Because of this, the limit of concentration in the atmosphere of, for instance, monomers of toluenediisocyanate, diphenylmethanediisocyanate, etc., was fixed at 0.02 ppm by the Commission of the American Conference of Governmental Industrial Hygienists. For such reason, for one thing, the aforesaid isocyanate monomers are seldom used as they are in polyurethane coatings, except in special cases, but are generally used after having been modified into prepolymers--adducts obtained by adding them to trimethylolpropane, ethylene glycol, etc. This modification has the effect of lowering vapor pressure, thereby reducing toxicity and bad odor, and, in addition, of allowing the adjustment of reactivity to be made and the diversification of the type of coatings to be realized.
Since, however, it is industrially extremely difficult to wholly eliminate isocyanate monomers in the prepolymer additions, the fact is still that one smells a strong irritating odor while he is engaged in the work of preparing coating materials or of applying coats, many people complaining of the symptoms of respiratory diseases peculiarly contracted by inhaling isocyanate vapor, and with an increase in the use of polyurethane products, this question has been brought much to the fore.
With the isocyanate additions, in particular, it is said that there occurs dissociation of diisocyanates--highly toxic monomers--while they are in store, depending upon the storage conditions, and this constitutes an uneasy factor for those concerned--chemical engineers and operators. Under such circumstances, measures are being taken for improvement of the working environment, for instance, ensuring a good ventilation, so that the operators will not directly inhale vapor of isocyanate compounds. However, the state of things in this connection is still far from being satisfactory.
The second question in the conventional technology is that there remains, with the conventional coatings, much to be desired in respect of their weather resistance. Although the non-yellowing type polyurethane resins were originally developed with a view to improving the weather resistance of coatings, they are not, as yet, sufficiently resistant to weather when used as coating for application to those commodities which are exposed to severe conditions outdoors over a long period of time, such as automobiles, railroad carriages, aircrafts, vessels, building materials, and so forth.
The third question concerning polyurethane coating materials according to the conventional techniques is that the range in which the selection of solvent composition can be made is not necessarily wide enough, and that, if the amount of solvent is reduced from the viewpoint of energy saving and prevention of environmental pollution, the resultant product being the so-called "high solid type" coating with a high concentration of polyurethane resin, the coating work efficiency is much impaired because of its high viscosity.